Yeah they are interesting to be on the list.
From what I can bather the EU guidelines don’t demand a high incidence but that they are capable of causing severe allergic reactions, that they not infrequently used in foods, and that it might not be otherwise obvious to consumers that it is in there. Risk based more than prevalence based.
And, importantly, is tested to not contain more than 20 ppm of gluten, since it can also show up just from harvesting. They don’t just assume.
I had a friend, who died a few years ago, who vomited immediately upon ingesting ANYTHING with coconut in it. One time, she was at a friend’s house and this happened after taking the first bite of something that was on the table, and asked the host if it contained coconut. She replied, “No, but it has coconut oil in it, and that’s healthy.” Not for her, it wasn’t!
Sulphite allergies are like that; sulphites denature proteins, including some of the proteins in your body when you eat or drink things containing sulphites (either naturally occurring or added as preservatives) - and it’s possible to acquire an allergy to one of those denatured proteins; basically a sulphite-moderated autoimmune disease.
I think I do have an allergy to broccoli.
Every time I have tried to eat it I’ve gotten sick and wanted to puke 
and felt like I was going to die! Just the smell of it cooking (like the stench of a well soiled diaper) makes me gag.
How is this different from any other food allergy? 
I’ve coined a word for anything I find offensive. “Broc”.
Allergies are typically indicated by an immune response by IgE antibodies, often resulting in hives, anaphylaxis etc.
Vomiting or feeling sick after eating a specific thing may be a food sensitivity - often caused by a digestive reaction where (for example) there is a problem with a missing enzyme or something, so something in the food can’t be properly broken down.
Pine nuts make me projectile-vomit about half of the times I’ve ever eaten them; it’s not an allergy and is most likely a food sensitivity. In a restaurant-ordering context, I wouldn’t bother differentiating though.
I can’t count the number of times when, telling someone I have a life-threatening allergy to tomatoes, they scoff and say “well - I never heard of that!” Well, maybe not, but they’ve definitely sent me to the hospital twice.
(Which is not to say anyone here is scoffing, just that I run into that)
Some allergies are more common than others, but at this point I think it is possible to be allergic to almost anything that has some form of protein in it, which would include anything living or formerly living that hasn’t turned entirely to rock.
Some things are more of a problem because they seem to be in everything. Like corn protein or gluten or milk components like whey or casein.
Then there is the whole confusion between “allergy”, “intolerance”, specific conditions like celiac…the tick-bite acquired allergy to alpha-gal protein… it can get quite complicated and confusing.
During my first career as a Sheriffs Deputy I pissed the wrong captain off and found myself working in the jail for almost 2 years.
I had inmates in my pod with all sorts of food allergies. Tomatoes were one of the common ones.
Onions, mustard, citrus, even peppermint.
I have an allergy to dairy protein but it’s only if I consume it. There was an inmate that couldn’t even touch milk or cheese or he’d need an epi pen. Just from skin contact.
Yeah, I’ve had people tell me my wife’s sulphite allergy isn’t possible, because sulphites aren’t proteins and you can only be allergic to proteins. Fortunately those have all just been internet keyboard warriors, not the other kind of idiot where they slip some of the problematic food in to supposedly ‘prove’ the allergy isn’t real.
FWIW true sulfite allergy is very rare. Generally it is an intolerance based on deficient sulfite oxidase, or a direct irritant effect, not immune mediated. There is some work on the modifying the body’s own proteins which then cause a reaction for some skin reactions, but it is rare. But yeah, a normal person would just say they’re allergic! 
My wife has the real allergy - anaphylaxis, hospitalisation, intubation, the works!
NOT doubting the severity and seriousness of her reaction. Just being a pedantic asshole! The experts tell us that that sort of life threatening anaphylaxis from sulfite very rarely is IgE mediated allergy as its mechanism.
More thought of as direct trigger of degranulation of mast cells with several not allergy mechanisms suggested.
It is severe. Life threatening. Asthma, anaphylaxis. Very scary. But skips the what most experts consider the allergy pathway to direct massive histamine release.
My father claimed that it’s technically possible to be allergic to anything. (Although you wouldn’t survive long enough to be here if you were allergic to water, or salt, it other extremely common substances.) It’s just that some allergies are common and others are extremely rare.
Neat. I am also allergic to tomatoes and it took my parents the better part of a year working with the pediatrician to figure that one out. Very rare, and I’ve never encountered anybody who shares it
I didn’t realize tomato allergies were uncommon. A cousin I spent a lot of time with while growing up is allergic, so to me it always seemed routine. Poor guy - he really liked tomatoes (or thought he did, anyway; I guess anything is more desirable when you can’t have it).
The real tragedy; your cousin can’t eat pizza.
Depends. With fruit/vegetable allergies, avoiding the seeds and skin and cooking the heck out of the rest can break the relevant proteins down enough to safely eat for many. This is the case for me. So, ketchup, which is highly processed, is generally safe for me to eat. But I’m not going to touch a pizza that has sliced tomatoes laid on top
The bit about Pollen Food Allergy Syndrome (PFAS) already mentioned is worth highlighting here:
Aquagenic urticaria.
When my brother was in grade school there was a boy in his class that would get severe hives and even breathing problems if he got soaked in water.
He was only in his 3rd grade class and then his family moved away. The following year there was a rumour that he died but that was never verified.
There are plenty of people who have lower level but unwanted reactions to things. Sometimes it’s like your body can cope with being exposed to THIS much of a substance without a reaction, but if you get too much, or a lower level of exposure but too frequently, then your body does things that make you regret it.
For example, one of my sisters sometimes has a reaction to tomatoes, but only when she’s been eating raw ones often. Cooked tomatoes, like in spaghetti or pizza sauce or ketchup are fine. But start eating raw ones frequently? Then she gets little raised red bumps all over her fingers which sort of look like pimples but itch terribly. Our family always grew tomatoes and we all loved them – summer lunches and suppers often were based on BLTs… to her eventual misery.
I can’t handle a lot of raw cucumber. A few slices in a mixed greens salad? Perfectly fine. A whole slew of sliced raw cucumbers in a yogurt dressing as a salad in itself? It starts me burping within a few minutes, and the burping goes on relentlessly for hours.
Weirdly, if the cucumbers have been turned into pickles they never bother me at all. And it’s not just the cooking factor that has been mentioned. If you simply do a ‘quick pickle’, basically letting the sliced up cucumbers sit in a water/vinegar solution for an hour or so, that’s enough to make them safe for me.